1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for an internal combustion engine having an evaporative fuel-emission suppression system, and more particularly to an evaporative fuel-purging control system of this kind which controls the flow rate at which evaporative fuel is purged into the intake system of the engine.
2. Prior Art
Conventionally, evaporative fuel-emission suppression systems have been widely used in internal combustion engines, which operate to prevent evaporative fuel from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of an air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged amount is large, the air-fuel ratio of the mixture fluctuates. To prevent such fluctuations, there have been proposed the following systems:
(i) a purging gas flow rate control system which reduces the purging amount from the start of the engine immediately after refueling or fill-up until the speed of a vehicle in which the engine is installed reaches a predetermined value, and also reduce the purging amount after the vehicle speed has reached the predetermined value and until the accumulated time period during which the vehicle speed exceeds a predetermined value, to thereby prevent fluctuations in the air-fuel ratio due to purging immediately after a fill-up when a large amount of fuel vapor can be produced in the fuel tank (e.g. Japanese Provisional Patent Publication (Kokai) No. 63-111277);
(ii) an air-fuel ratio control system which effects purging of evaporative fuel in such a small amount as to cause almost no fluctuation of the air-fuel ratio, detects an amount of variation of an air-fuel ratio correction coefficient applied to feedback control of the air-fuel ratio, which is caused by the purging, forecast from the detected variation amount a value of the air-fuel ratio correction coefficient which should be assumed when the purging amount is large, and applies the forecast value as the air-fuel ratio correction coefficient in the feedback control when the actual purging amount becomes large, so as to reduce the fuel amount supplied to the engine, whereby fluctuations in the air-fuel ratio can be suppressed even when the purging amount is large (e.g. Japanese Provisional Patent Publication (Kokai) No. 62-131962;
(iii) a purging gas flow rate control system which controls the purging amount by the use of an output from an exhaust gas ingredient concentration sensor provided in the exhaust system of an internal combustion engine, or an air-fuel ratio correction coefficient calculated from the sensor output (e.g. Japanese Provisional Patent Publications (Kokai) Nos. 57-129247, 58-30458, 61-129454, 62-233466, and 63-85249.
However, the above system (i) merely reduces the purging amount under predetermined conditions determined by the vehicle speed and the predetermined time period after a fill-up, but does not effect control of the purging amount based upon the actual amount of fuel vapor gas (vapor amount). Therefore, since the vapor amount is unknown when a long time period has elapsed after a fill-up, and the actual vapor amount is different depending upon the residual fuel amount in the fuel tank even immediately after a fill-up, it is impossible to eliminate fluctuations in the air-fuel ratio and enable the evaporative fuel-emission suppression system to exhibit its full suppressing capacity, at the same time. That is, generally in conventional evaporative fuel-emission suppression systems, to prevent fluctuations in the air-fuel ratio, the purging amount is set to a moderate amount such that the width of air-fuel ratio fluctuation is kept within an allowable range even when the maximum vapor amount is produced, and therefore the total purging amount is limited.
The system (ii), which controls the fuel supply amount by the use of the air-fuel ratio correction coefficient reflecting the actual purging flow rate, can suppress fluctuations in the air-fuel ratio to be caused by purging. However, this system does not control the purging amount in response to the forecast air-fuel ratio correction coefficient. As a result, the value of the correction coefficient becomes largely deviated from a central value thereof when the purging amount is large. Particularly, when the air-fuel control has shifted from an open loop control mode to a feedback control mode, an average value of the air-fuel ratio correction coefficient, which is used as an initial value of the correction coefficient at the start of the air-fuel ratio feedback control, becomes largely deviated from the central value. If the deviated average value is actually used in the air-fuel ratio feedback control, it can result in a degradation in the control responsiveness.
Further, to operate the evaporative fuel-emission suppression system at its full capacity in order to fully suppress emission of evaporative fuel gas into the atmosphere, it is desirable to purge an amount of evaporative fuel as large as possible insofar as the air-fuel ratio fluctuation is kept within the allowable range. However, if in the systems (i) and (ii), the purging flow rate is set to such a value that the air-fuel ratio fluctuation is kept within the allowable range even if the vapor amount assumes the maximum value, the total purging amount cannot be sufficient enough to operate the evaporative fuel-emission suppression system at its full capacity even when the vapor amount is small, since the purging flow rate is thus set to the above value, though the purging amount can be increased insofar as the air-fuel ratio fluctuation lies within the allowable range, whereby the evaporative fuel-emission suppression system cannot be operated to its full capacity.
A system disclosed in Japanese Provisional Patent Publication (Kokai) No. 62-233466 amongst the systems (iii), employs a plurality of purge control valves, and calculates a forecast value of an air-fuel ratio correction coefficient to be applied during large-amount purging, based upon values of the correction coefficient during stoppage of the purging and during small-amount purging, and inhibits large-amount purging when the forecast value exceeds a predetermined value.
However, in general, the air-fuel ratio correction coefficient largely varies depending upon engine operating parameters such as intake pipe pressure, engine rotational speed, engine temperature, and intake temperature. Further, even if these parameters remain unchanged, the correction coefficient incessantly varies. Therefore, the forecast value varies with these parameters, and if the correction coefficient varies during purging stoppage and during small-amount purging, the resulting forecast value largely varies by an amount several times as large as the variation amount of the correction coefficient during purging stoppage or during small-amount purging, so that the forecast value cannot accurately show a value of the correction coefficient to be assumed during large-amount purging. As a result, there is a possibility that large-amount purging is inhibited even when it can be actually effected. Thus, the system disclosed in Publication No. 62-23346 is also unable to enable the evaporative fuel-emission suppression system to exhibit its full capacity, like the systems (i), (ii).